Each turbine, or turbine section, is understood to be a steam turbine within the meaning of the present application, which is exposed to throughflow by a working medium in the form of steam. In contrast to this, gas turbines are exposed to throughflow by gas and/or air as working medium which, however, is subjected to completely different temperature and pressure conditions than the steam in a steam turbine. As opposed to gas turbines, in steam turbines, for example the working medium which flows in a turbine section at the highest temperature, simultaneously has the highest pressure. An open cooling system, which is open to the flow passage, is realizable in gas turbines even without external feed for cooling medium to turbine sections. For a steam turbine, an external feed of cooling medium should be provided. The prior art with regard to gas turbines cannot be drawn upon for the assessment of the subject matter of the present application, if only for that reason.
A steam turbine customarily comprises a rotatably mounted rotor which is populated with blades, which is installed inside a casing or casing shell, as the case may be. During throughflow exposure of the interior space of the flow passage, which is formed by the casing shell, to heated and pressurized steam, the rotor, via the blades, is set in rotation by means of the steam. The blades of the rotor are also referred to as rotor blades. Furthermore, stationary stator blades are customarily suspended on the inner casing, which stator blades reach into the interspaces of the rotor blades along an axial extent of the body. A stator blade is customarily mounted at a first point along an inner side of the steam turbine casing. In this case, it is customarily part of a stator blade row which comprises a number of stator blades which are arranged along an inside circumference on the inner side of the steam turbine casing. In this case, each stator blade points radially inwards with its blade airfoil. A stator blade row at the first point which was mentioned along the axial extent is also referred to as a stator blade cascade or stator blade ring. A number of stator blade rows is customarily connected one after the other. At a second point along the axial extent, behind the first point, a second further blade is correspondingly mounted along the inner side of the steam turbine casing. A pair of one stator blade row and one rotor blade row is also referred to as a blade stage.
The casing shell of such a steam turbine can be formed from a number of casing segments. Especially the stationary casing component of a steam turbine, or turbine section, is understood to be the casing shell of the steam turbine, which along the longitudinal direction of the steam turbine has an interior space in the form of a flow passage which is provided for throughflow exposure to the working medium in the form of steam. Depending upon the type of steam turbine, this can be an inner casing and/or a stator blade carrier. However, a turbine casing can also be provided which has no inner casing or no stator blade carrier.
For reasons of efficiency, the design of such a steam turbine for so-called “high steam parameters”, therefore especially high steam pressures and/or high steam temperatures, can be desirable. However, especially a temperature increase is not indefinitely possible for material engineering reasons. To enable a safe operation of the steam turbine in this case, even at especially high temperatures, a cooling of individual component parts or components, therefore, can be desirable. The component parts are specifically limited in their resistance to temperature. Without efficient cooling, significantly more expensive materials (for example, nickel based alloys) would be necessary in the case of increasing temperatures.
In the hitherto known cooling methods, especially for a steam turbine body in the form of a steam turbine casing or a rotor, a distinction is to be made between an active cooling and a passive cooling. In an active cooling, a cooling is effected by means of a cooling medium which is fed separately to the steam turbine body, i.e. in addition to the working medium. In contrast, a passive cooling takes place purely by means of a suitable guiding or application of the working medium. Up to now, steam turbine bodies were preferably passively cooled.
In this way, for example from DE 34 21 067 C2, it is known to circulate cool, already expanded steam around an inner casing of a steam turbine. However, this has the disadvantage that a temperature difference over the inner casing wall must remain limited since otherwise with too great a temperature difference the inner casing would be too severely thermally deformed. During a circulating of flow around the inner casing, in fact a heat discharge takes place, however, the heat discharge takes place relatively far away from the point of the heat feed. A heat discharge in the direct vicinity of the heat feed has not been put into effect in sufficient measure up to now. A further passive cooling, by means of a suitable design of the expansion of the working medium can be achieved in a so-called diagonal stage. By this, however, only a very limited cooling action for the casing can be achieved.
An active cooling of individual components inside a steam turbine casing is described in U.S. Pat. No. 6,102,654, wherein the cooling is limited to the inflow region of the hot working medium. Some of the cooling medium is added to the working medium. In this case, the cooling is to be achieved by a flow-washing of the components to be cooled.
From WO 97/49901 and WO 97/49900 it is known to selectively charge an individual stator blade ring, for shielding of individual rotor sections, with a medium by means of a separate radial passage in the rotor, which is fed from a central chamber. For this purpose, the medium is added to the working medium via the passage and the stator blade ring is selectively flow-washed. In the center hollow bore of the rotor which: is provided for this, however, increased centrifugal force stresses are to be taken into account, which represents a considerable disadvantage in design and operation.
A steam turbine with a balance piston is disclosed in U.S. Pat. No. 3,614,255, wherein the balance piston is exposed to steam flow which flows from a line which leads into the flow passage downstream of a blade row. A single-flow steam turbine with a balance piston is disclosed in U.S. Pat. No. 4,661,043, wherein the balance piston is cooled. A single-flow steam turbine with a balance piston is disclosed in U.S. Pat. No. 2,796,231, which balance piston is exposed to cooling steam flow via a line which is located in the inner casing.
A possibility for extraction and guiding of a cooling medium from other areas of a steam system, and feed of the cooling medium in the inflow region of the working medium, is described in EP 1 154 123.
To achieve higher efficiencies in current generation with fossil fuels, the requirement exists to use higher steam parameters, i.e. higher pressures and temperatures in a turbine, than were customary up to now. In the case of high temperature steam turbines, with steam as the working medium, temperatures in part well above 500° C., especially above 540° C., are provided. Such steam parameters for high temperature steam turbines are disclosed in detail in the article “New Steam Turbine Concepts for Higher Inlet Parameters and Longer End Blades” by H. G. Neft and G. Franconville in the magazine VGB Power Plant Technology, Nr. 73 (1993), issue 5. The disclosure content of the article is introduced herewith in the description of this application in order to disclose different embodiments of a high temperature steam turbine. Examples of higher steam parameters for high temperature steam turbines are especially referred to in FIG. 13 of the article. In the article which is referred to, a cooling steam feed and transmission of the cooling steam through the first stator blade row is proposed for improving the cooling of a high temperature steam turbine casing. By this, an active cooling is indeed made available. However, this is limited to the main flow region of the working medium and is still worthy of improvement.
All cooling methods which are known for a steam turbine casing up to now, therefore, in so far as they are principally active cooling methods, provide in any event a concentrated flow-washing of a separate turbine section which is to be cooled, and are limited to the inflow region of the working medium, in any event with inclusion of the first stator blade ring. During a loading of conventional steam turbines with higher steam parameters, this can lead to an increased thermal loading which affects the whole turbine, which could be only inadequately reduced by means of a customary cooling of the casing which is described above. Steam turbines which, for achieving higher efficiencies, operate principally with higher steam parameters, require an improved cooling, especially cooling of the casing and/or the rotor, in order to relax a higher thermal loading of the steam turbine in sufficient measure. In this case, there is the problem that during the use of hitherto customary turbine materials, the increasing stress of the steam turbine body as a result of increased steam parameters, for example according to the “Neft” article, can lead to a disadvantageous thermal loading of the steam turbine. Consequently, a production of this steam turbine is hardly possible anymore.
An effective cooling is desirable in a steam turbine component, especially for a steam turbine which is operated in the high temperature range.